This application relates generally to disc drive data storage devices and more particularly to an apparatus and method of writing servo track information thereon.
Disc drives are the most common means of storing electronic information in use today. Ordinary disc drives are typically constructed with the following internal components: one or more magnetic media discs attached to a spindle; a spindle motor that rotates the spindle and the attached discs at a constant high speed; an actuator assembly, located adjacent to the discs, with a plurality of actuator arms that extend over the discs, each with one or more flexures extending from the end of each actuator arm, and with a read/write head mounted at the distal end of each flexure; and a servo positioner that rotates the actuator assembly about a bearing shaft assembly positioned adjacent to the discs such that the read/write heads radially traverse the disc surface (i.e., move back and forth the between the inner and outer diameters of the disc).
Information is stored on and retrieved from a magnetizable material on the disc""s surface. To facilitate information storage and retrieval, discs are radially divided in concentric circles known as xe2x80x9cservo tracksxe2x80x9d or xe2x80x9ctracksxe2x80x9d. The tracks are given a track number so that the servo positioner can locate a specific track. The servo positioner, upon receiving a control command, aligns the read/write head over the desired track. Information can be stored or retrieved from the disc once the read/write head is in the correct position. The process of switching between different tracks is called xe2x80x9cseekingxe2x80x9d, whereas remaining over a single track while information is stored or retrieved is called xe2x80x9cfollowingxe2x80x9d.
Each track is subdivided into pie-shaped sections, called xe2x80x9csegmentsxe2x80x9d or xe2x80x9csectorsxe2x80x9d. The two most common types of sectors are informational data sectors and servo data sectors. In a typical disc drive, the informational data sectors usually contain information generated or stored by the user such as programs files, application files, or database files. There may be ten to a hundred, or even more, informational data sectors dispersed around a single track.
The servo data sectors, on the other hand, contain information that is used by the servo positioner to determine the radial position of the head relative to the disc surface and relative to the track center. Servo sectors typically consist of a Grey code field, which provides coarse position information such as the track and cylinder number, and a servo burst field, which provides fine position information such as tie relative position of the head to the track center. Generally speaking, the burst field creates a signal with a specific voltage magnitude when the read head is not aligned over the track centerline. The signal is decoded to pinpoint the read head""s location and the read head is moved directly over the centerline by positioning the read head such that the sum of the burst field voltages equal zero.
Servo sectors are usually placed between adjacent informational data sectors on the same track. A clock signal mechanism is used to insure that data intended to be stored in a servo sector does not overwrite data in an information sector (and vice versa).
During the servo writing process, a timing pulse from the clock signal mechanism notifies the servo positioner when the head is over a servo sector (as opposed to over an information sector). The write enable signal is turned on and information is written to the servo sector. The timing pulse also notifies the servo positioner when the head is over an information sector. The write enable signal is turned off and servo information is not stored in the informational data sector during the servo writing process.
In contrast during normal disc drive operation, the timing pulse notifies the servo positioner when the head is over an information sector (as opposed to a servo sector). The write enable signal is turned on and data is written to the information sector. The timing pulse also notifies the servo positioner when the head is over a servo sector. The write enable signal is turned off and user data is not stored in the servo sector during normal disc drive operation.
Information is transferred to and from the tracks by the read/write heads attached to the flexures at the end of the actuator arms. Each head includes an air bearing slider that enables the head to fly on a cushion of air in close proximity to the corresponding surface of the associated disc. Most heads have a write element and a read element. The write element is used to store information to the disc, whereas the read element is used to retrieve information from the disc.
The number of tracks located within a specific area of the disc is called the xe2x80x9ctrack densityxe2x80x9d. The greater the number of tracks per area, the greater the track density. The track density may vary as the disc is radially traversed. Disc manufacturers attempt to increase track density in order to place more information on a constant size disc. Track density may be increased by either decreasing the track width or by decreasing the spacing between adjacent tracks.
An increase in track density necessitates increased positioning accuracy of the read/write elements in order to prevent data from being read from or written to the wrong track. Manufacturers attempt to fly the read/write head elements directly over the center of the desired track when the read/write operation occurs to insure that the information is being read from and written to the correct track. Hitting the track center target at high track densities requires that the tracks be as close to perfectly circular as possible when written to the disc surface.
Tracks are usually written on the disc during manufacturing using one of two means: 1) a servowriting machine, or 2) self-propagated servo writing. In both methods, a timing clock is used to notify the servo positioner when the head is over an area where a servo sector is to be written. A write enable signal is activated and servo information is written when the timing pulse indicates that the head is located over a servo sector. The write enable signal is de-activated and information is not written once the head exits the area where a servo sector is to be written.
A servowriting machine is a large piece of external equipment that writes servo tracks on a disc drive. The servowriting machine uses a very accurate lead screw and laser displacement measurement feedback device to precisely align a write element. The write element, which is attached to an external head/arm positioner, is aligned relative to where the desired track is to be written on the disc surface. A track is written on the disc once the write element is correctly aligned. The head/arm positioner then moves the write element a predetermined distance to the next desired track location. The head/arm positioner, therefore, controls both the track placement and track-to-track spacing.
A servowriter, however, has several drawbacks. First, a typical disc may contain more than 60,000 servo tracks. The process of aligning and writing each track on the disc is very time consuming and expensive. Second, although very accurate at lower track densities, the servowriter cannot meet the accuracy requirements dictated by higher track densities. Finally, the procedure must be completed in a clean room because the disc components are exposed during servowriting; again adding expense to the servowriting procedure.
The second means of writing tracks on a disc is called self-propagating servo writing. Oliver et al first described this method of servo track writing in U.S. Pat. No. 4,414,589. Several other patents have disclosed slight variations in the Oliver patent, but the same basic approach is used. Under the basic method, the drive""s actuator arm is positioned at one of its travel range limit stops. A first reference track is written with the write head element. The first reference track is then read with the read element as the head is radially displaced from the first reference track. When a distance is reached such that the read element senses a predetermined percentage of the first reference track""s amplitude, a second reference track is written. The predetermined percentage is called the xe2x80x9creduction numberxe2x80x9d. For example, the read element senses 100% of the first reference track""s amplitude when the read element is directly over the first reference track. If the reduction number is 40%, the head is radially displaced from the first reference track until the read element senses only 40% of the first reference track""s amplitude. A second reference pattern is written to the disc once the 40% is sensed by the read element. The head is then displaced in the same direction until the read head senses 40% of the second reference track""s amplitude. A third reference track is then written and the process continues. The process ends when the actuator arm""s second limit stop is reached and the entire disc surface is filled with reference tracks. The average track density is then calculated using the number of tracks written and the length of travel of the head.
If the average track density is too high, the disc is erased, the reduction number is lowered so that a larger displacement occurs between tracks, and the process is repeated. If the track density is too low, the disc is erased, the reduction number is increased so that a smaller displacement occurs between tracks, and the process is repeated. If the track density is within the desired range, the reduction number for the desired average track density has been determined, the disc is erased, and servo tracks are written to the disc by alternatively writing servo and reference tracks. The servo tracks are further divided by alternatively writing servo and informational sectors.
Several drawbacks exist with the current methods of self-propagating servo writing. First, the disc has to be free of xe2x80x9cnoisexe2x80x9d (i.e., stray signals) before the first servo track can be written on the disc. A disc erase procedure must be completed to rid the disc of noise. The erase process is completed by moving the actuator assembly to one of its travel-range-limit stops, generating a write enable signal in the write element without a corresponding write data signal, and xe2x80x9cspiralingxe2x80x9d the head over the disc surface until the actuator assembly reaches its second travel stop. This process is xe2x80x9cblindxe2x80x9d in the sense that the position of the head cannot be accurately determined because all information, even servo information, is erased as the write head spirals over the disc surface. As a result, it is likely that the disc has not been completely erased and some signal noise remains.
Noise reduces the accuracy of the servo writing process by causing the read head to erroneously determine that the reduction number has or has not been met. The read element, for example, may be at a distance from the reference track where, without noise, 40% of the reference track""s signal would be sensed. The read element, however, senses a different amplitude, say 50% of the reference track signal""s amplitude, because of noise present on the magnetic media. The servo positioner adjusts the actuator arm to compensate for the noise until the read head senses 40%. The read head will then be at an incorrect distance from the reference track and the track that is being written contains a shape defect.
A second drawback of self-propagating servo writing is that it is difficult to obtain perfectly circular tracks. Mechanical problems (such as spindle wobble, disc slip, changing head fly height, and thermal expansion among others) will cause imperfections in the shape of the track being written. For example if the fly height of the read element increases, the signal strength of the previous track will decrease below the desired level and the actuator arm is moved to compensate. This adjustment causes a deformation in the track that is being written. Any imperfections in the previous track will propagate to the track being written because the read element xe2x80x9cfollowsxe2x80x9d the path of the previous track in self-propagating servo writing. The imperfections of the previous track may even be amplified within the new track in some circumstances.
Another problem with prior art self-propagating servo writing is caused by the changing virtual read/write element offset. The virtual offset of the read element to the write element, relative to the surface of the disc, changes as the head radially traverses the disc. The virtual offset between the read/write elements approaches or equals zero (i.e., the read and write elements line up) at certain actuator arm positions. The head position signal, derived by monitoring the magnitude of the signal generated in the read head by the previously written track, is difficult to obtain at the points where the virtual offset approaches or equals zero. This approach to measure position is inherently inaccurate due to signal magnitude variations caused by changing fly height and media imperfections. The servo positioner, therefore, cannot accurately place the head relative to the disc surface and errors in the servo writing occur when the virtual offset approaches or equals zero and an accurate position error signal is not available.
Ideally, tracks are perfectly circular and spaced at a specific distance from each other. Imperfections in track shape and spacing are referred to as xe2x80x9ctrack squeezexe2x80x9d. Track shape imperfections are referred to as dynamic or AC track squeeze, whereas track spacing imperfections are referred to as static or DC track squeeze. AC track squeeze refers to the situation in which two adjacent tracks have shape imperfections at different locations around their individual circumferences. The two tracks may be too close together at some points and too far apart at other points. DC track squeeze, on the other hand, refers to the situation in which two adjacent tracks are either closer or farther apart than a nominal distance. In other words, the spacing between the two tracks is incorrect even though the two tracks are perfectly circular. The term xe2x80x9ctrack squeezexe2x80x9d is often used to generally refer to the combination of AC and DC track squeeze. Furthermore, the track-to-track variation of track shape is called the xe2x80x9crelative track shape errorxe2x80x9d, whereas the deviation of the track shape from a perfect circle is called xe2x80x9cabsolute track shape errorxe2x80x9d. The prior art methods of machine servo writing and self-propagated servo writing cannot achieve the accuracy needed for higher track densities because of inherent limitations in controlling track squeeze, relative track shape error, and absolute track shape error.
The read/write head elements, as mentioned above, are targeted to fly directly over the center of the desired track when the read/write operation occurs to insure that the information is being read from and written to the correct track. Track shape imperfections make continuous centering of the read/write elements over the track difficult. A method of compensating for imperfections in track shape, called Zero Acceleration Path (xe2x80x9cZAPxe2x80x9d) correction, has been developed. The basic idea of ZAP correction is to add appropriate correction factors to the measured head position at each servo sector. The correction factors cancel all written in errors, thereby resulting in a nearly perfectly circular modified track. The correction factors are typically determined during or after the servo track writing process. The correction factors are then written back on the disks; usually each servo sector has a dedicated field for storing the correction factors. The prior art methods of self-propagated servo writing, however, do not use ZAP correction to prevent track shape imperfections during the self-propagating servo writing process.
Accordingly there is a need for a method of writing servo tracks on disc drives that overcomes the limitations of both a prior art servowriter and self-propagating servo writing.
Against this backdrop the present invention has been developed. The present invention proposes a new servo track writing technique called Extended Copying with Head Offset (xe2x80x9cECHOxe2x80x9d). In a preferred embodiment of the ECHO technique, the read and write elements of the read/write head are offset from each other by at least one track width. In other words, if the read head is aligned with a first track, the offset is at least large enough that the write head is aligned over the adjacent track. In accordance with a preferred embodiment of the present invention, the read head is closer to the inner diameter and the write head is offset at least one track width towards the outer diameter relative to the disc""s surface. However, the method is also applicable when the read head is closer to the outer diameter of the disk. It should also be noted that the ECHO technique may be implemented using any clock-timing signal propagation methods that are described in prior art self-servo track writing approaches.
In a preferred embodiment of the ECHO technique, a disc is placed in a servowriter and a guide pattern is written on the inner diameter of the magnetic media disc. The guide pattern is comprised of servo tracks written on the disc. The guide pattern should contain at least as many tracks as the reader-writer offset. Preferably the guide pattern is comprised of 50 to 100 servo tracks, as compared to the more than 60,000 servo tracks written by a servo writer using prior art methods. An enormous amount of time is saved because only a guide pattern is written with the servowriter. Additionally, the accuracy limitations encountered by the servowriter machine at high track densities is avoided. Furthermore, the amount of xe2x80x9cclean roomxe2x80x9d time required is decreased, thereby reducing the overall cost.
The disc is then mated with a head actuator assembly to form a head disc assembly (xe2x80x9cHDAxe2x80x9d). The HDA is connected to an electrical control system for self-propagating servo writing. ZAP correction factors are determined and written to the servo sectors of the guide pattern. The self-propagating servo writing begins after the ZAP correction factors are added to the guide pattern.
Self-propagating servo writing begins with the control system completing several calibrations (such as calculating reader/writer offset, writer width, and read width among others). The control system displaces the actuator arm until the read element is located over the position in the guide zone such that the write element, offset from the read element, is aligned over the location to which the next servo track is to be written. Preferably the read element is at least one track closer to the inner diameter of the disc surface because the read and write elements are offset. The write element is then activated and a new servo track is written by the write element. ZAP correction factors are then added to the servo sectors of the newly written servo track.
The control system then displaces the actuator arm until the read element is located at the position over the previously written tracks (i.e., either within the guide zone or over the self-propagated tracks) such that the write element, offset from the read element, is aligned over the location to which the next servo track to be written. The read head is at least one track closer to the inner diameter of the disc surface because of the offset. The write element is then activated and a new servo track is written. Again, ZAP correction factors are added to the servo sectors of the newly written servo track. Again, the control system, using the calculated offset, displaces the actuator arm until the write element is aligned with the next servo track to be written. The process continues until the desired number of servo tracks is written on the disc, the disc is filled with servo tracks, or the actuator assembly travel-limit-stop is reached.
In another preferred embodiment, the read element follows the guide pattern servo tracks, and then, the servo tracks written by the write element during the ECHO process. It is not necessary for the read/write element offset to be a whole number of tracks. In other words, the read element will not necessarily be positioned over a track center during the ECHO servowriting process. A 3-xc2xd-track reader/writer offset, for example, will cause the read element to be located over a boundary between two previously written tracks when the write element is located over the center of the track being written.
The ECHO technique typically eliminates the need to completely erase the disc as required by the previous methods of self-propagating servo writing. The read element always follows the write element during servo writing because of the read/write element offset. As a result, the write head erases any noise present on the disc surface as it writes new servo tracks. Furthermore, the write element is not completing a xe2x80x9cblindxe2x80x9d spiraling erase, as is the case in the prior art, because the read element obtains an accurate position feedback signal for the control electronics.
The present invention also eliminates many of the mechanical errors that affect the accuracy of prior art self-servo writing. As previously stated, the prior art method displaces the read head until the signal strength of the previously written servo track is detected. The accuracy of reading a percentage of the signal strength of the previously written servo track is affected by mechanic problems such as spindle wobble, disc slip, changing head fly height and thermal expansion among others. The present invention eliminates this problem by using the same positioning method that is used during normal disc operation. The more accurate servo positioning keeps the read head better aligned. As a result, the track that is being written is less likely to contain deformations caused by mechanical problems.
Furthermore, the present invention eliminates position control limitations caused by changing virtual offset that adversely affect prior art self propagated servo track writing methods. The virtual offset between heads changes as the head radially traverses the disc surface. At some point, the read and write heads are xe2x80x9cvirtually alignedxe2x80x9d and the accuracy of the position error signal decreases. The present invention eliminates this problem because the offset is large enough such that the read and write elements never xe2x80x9cvirtually alignxe2x80x9d. Therefore, an accurate position error signal is always available during the servo track writing process. Preferably the change in virtual offset is accounted for by calibrating the control system (such as calculating the virtual reader/writer offset among others) at different locations relative to the disc surface.
A preferred embodiment of the present invention utilizes ZAP correction factors to cancel any written errors that occur during the servo writing process. The ZAP correction factors can be used to correct errors in the guide pattern and to correct errors present in the servo tracks written by the offset write head. The ZAP correction factors are written into both the guide patterns and offset-write-head-written servo tracks before the actuator arm is displaced to align the offset read head. The read head""s track following ability is greatly improved by the ZAP correction factors, allowing the track being written by the offset write head to be nearly perfectly circular.
Finally, the present invention reduces AC and DC track squeeze and allows higher track densities to be achieved. The present invention eliminates AC and DC track squeeze by avoiding xe2x80x9cvirtual alignmentxe2x80x9d (thereby providing better position error signals during servo writing), by utilizing ZAP correction factors (thereby providing a nearly perfectly circular track for the read element to follow), and by avoiding the xe2x80x9cblindxe2x80x9d spiral erase process (thereby eliminating any noise that may cause deformation in the written servo track). These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.